How people work... and the mystery of your fingerprints

January 6, 2010

Why do we chew our food? Research has shown that it is not, as has long been presumed, to make chunks of food small enough to swallow without choking. Biomechanics, who have modelled the cohesive strength of food after a certain amount of chewing, have shown that we actually chew our food to ensure it is in a firm blob and, therefore, safe to swallow.

Writing in January's Physics World, Dr Roland Ennos, a biomechanic in the Faculty of Life Sciences at University of Manchester, explains how we need to look beyond obvious answers if we are to understand how our own bodies work.

Explaining why we swing our arms, why we have notched teeth, why our fingernails always break in the same direction, and, still puzzling, why we have fingerprints, Dr Ennos shows how rich the boundary between biology and physics is in, some counter-intuitive but, potentially significant discoveries.

On the fingerprint puzzle, we know that fingerprints are useful to identify people for security and crime detection, but no scientist has ever suggested that fingerprints evolved specifically for this purpose. It has been thought that fingerprints help us to grip more tightly to objects, but tests show that a rough surface does not actually increase the friction of soft materials such as skin.

Fingerprint friction is therefore a mystery that has left Dr Ennos's team testing a number of options - it could be that fingerprints act like the treads on tyres, removing water and so increasing friction under wet conditions. Another possibility is that prints also make the skin more flexible and stop it blistering.

As Dr Ennos writes, "The answers to these questions may appear obvious or even trivial, but further thought and experiment is revealing that our world is far more fascinating than we could have dreamed."

What's more, this sort of research, unlike many areas of physics, is not expensive or mathematically hard. "All you need is an enquiring mind, a bit of ingenuity and the courage to ask awkward questions," concludes Dr Ennos.

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"It has been thought that fingerprints help us to grip more tightly to objects, but tests show that a rough surface does not actually increase the friction of soft materials such as skin." - in DRY conditions

Like it suggests they may to improve friction when wet. When driving in wet conditions treads allow the water to escape. In rubber tyres the main mechanism for friction is adhesion and so maximum contact area is preferred and so treads are not used in dry conditions.

I always thought that fingerprints were an evolved way for the skin to grow and replace itself in a high-use and high-sensitivity application. The skin can continue to stay thick yet pliable without any changes in sensitivity. The usual flake away "system" used over the rest of the body would degrade too quickly under heavy use. And the leathery "system" (think chimps) would too greatly reduce sensitivity for our very fine human applications.

If you've ever had an injury that permanently removes some area of the fingerprint, you can observe how the resultant non-fingerprint skin has a difficult job replacing itself and maintaining pliancy and unchanging sensitivity.

They don't use race slicks to increase traction. They use race slicks to increase surface area.

VST is applied to the track for traction.

Race slicks both increase surface area and traction due to softer compounds. But the only reason for not having tread is to increase surface area. I believe it is VHT also, which is a mixture of alcohol and glue.

Race slicks both increase surface area and traction due to softer compounds. But the only reason for not having tread is to increase surface area. I believe it is VHT also, which is a mixture of alcohol and glue.

Quite right. The additional surface area and overall larger size are primarily to address the huge heat differential and allow for thermal expansion in the tires. That's part of the reason why they might pull a car out of a race that's been in the pits for an extended period of time. It becomes less safe as the tires cool and the surface area as well as overall tire volume is reduced, in addition to how the friction of the braking system is greatly reduced by cooling.

Race slicks do not increase traction. The additional surface area and overall larger size is primarily to address the huge heat differential and allow for thermal expansion in the tires.

Different compounds have differing coefficients of friction, to simply state that differing tire compounds create no difference in traction aka your friction coefficient is simply wrong. Tires expanding due to thermal forcing and "gaining surface" area is absolutely not the reason for wanting heated tires, each compound has an ideal thermal range and in racing R-compound tires that temperature is very high which softens the compound and increases your friction coefficient. The thermal expansion of tires is barely measurable and is not a factor in racing.

Obviously I am disagreeing, just because the underlying physics use the same terminology doesnt mean you were using the term thermal expansion in the way I was. You were stating that the primary reason is due to surface area and volumetric area of the tire which is wrong. I've been racing for 15 years and have a garage full of race tires as well as street tires, you can take the identically sized tire of a standard compound vs race compound and your difference in traction is not even comparable.

There is a logical and energy conversing reason for it.As for chewing it must be fairly important because those muscles are your strongest. Just get a pit-bull to bit your arm for an impressive example.

Another completely different explanation for fingerprints lies in the incredible sensitivity of the fingertips (compared to any other skin area of similar size). By having a lot of nerves terminating in skin ridges, the fingertips can discern cracks and surface flaws of materials of less than .001 inch. For a tool maker and user, this is quite an evolutionary advantage.

Just take a look at how many different purposes our fingers serve. Looking further you will find that fingerprints somehow help them all. That's why they are not perfect for any of them. But I'm sure there is no better solution.

Just a small example: fingertips must be very good at gripping stuff with any natural texture, regardless wet or dry, cold or warm, and still they don't have to adhere (quick detaching). In contrast Rubber tires are designed for specific conditions - road type, temperature, wetness etc.

I've been racing for 15 years and have a garage full of race tires as well as street tires, you can take the identically sized tire of a standard compound vs race compound and your difference in traction is not even comparable.

And I am an automotive engineer. Thermal expansion of rubber makes it more porous as well as softer. Note I said rubber. Street tires have a lower rubber content than "race" tires. The tire face will adhere to the road better as there is more surface area in contact with the asphalt. That is what raises the coefficient of friction. The softness brought on by thermal expansion of the rubber allows the tire to sink into the imperfect surface. If the tires did not do this, the car would have far less contact with the road surface and the effective surface area providing contact, and therefore traction, would be greatly decreased.

My father, after he retired, set up a small workshop in the back of the house - where he would 'putter' away merrily for days.

eventually his fingers became hard and horny - he still had fingerprints, as in the grooves and ridges, but they were very hard.

It was alomst impossible for him to pick up a smooth piece of paper because his fingers had no 'give' to them. This sounds like the same situation as drag racers that have to have 'soft' rubber to get any traction.

I respectfully suggest that you are confusing surface area with contact area. The tire's warm and cold surface area are very close to equal. But versus the road surface, the warm tire (hopefully weighted) has a higher contact area due to its surface's greater pliability. Greater contact area = greater traction.

I think finger prints are evolved by female selection. For maximum sexual gratification during couplings. The male who has fine prints on the finger digits can likely bring the female higher sexual pleasure. You don't want the finger to be too rough nor too smooth!